Field of the Invention
The present invention relates to improvements of a lens meter which is capable of readily measuring the refraction characteristics of the progressive multifocal lens by means of the lens meter.
Recently, there has been a growing demand for progressive multifocal lenses having no boundary which are used for correction of the early-stage prebyopia of the middle aged and the elderly. Since this progressive multifocal lens is constructed such that a portion for the far distance, a portion for the near distance, and a progressive belt portion connecting the two portions are formed continuously on a nonspherical surface, it is impossible to identify a far-distance refraction characteristics measuring portion and a short-distance refraction characteristics measuring portion by observing its appearance only, as with a conventional bifocal lens. The term "refraction characteristics" referred to here is employed as a general term meaning the degree of spherical surface, the degree of cylinder, the angle of cylindrical axis, and the degree of prism. For this reason, to facilitate measurement of refraction characteristics and working of lenses at the time of fitting lenses in eyeglass frames at eyeglasses stores, lens manufacturers provide various marks, as shown in FIG. 1, on uncut lenses before they are fitted in the eyeglass frames, which are supplied to eyeglasses stores. FIG. 1 shows one example of such marks, in which reference numeral 10 denotes a horizontal reference line; 11, a diamond mark; 15, a mark indicating the geometrical center and optical center; 12, a fitting point; 13, a mark indicating a far-distance refraction characteristis-measuring portion; 16; a mark indicating near-distance refraction characteristics; 14, a mark indicating the degree of the near distance; and 17, a manufacturer's mark. At the time of measuring the far-distance refraction characteristics of this lens, the lens is set in such a manner that the measurement optical axis of the lens meter is located in the circle of the mark 13, and the mark 13 will coincide with the center of the lens holder of the lens meter. On the other hand, at the time of measuring the near-distance refraction characteristics, the lens is set in such a manner that the circle of the mark 16 coincides with the lens holder. In addition, if it is desirous to know the refraction characteristics at a fitting point, as necessary, the lens is set in such a manner that a point of intersection 12a of the mark 12 coincides with the center of the lens holder.
A conventional lens meter for progressive multifocal lenses having the aforementioned arrangement has had the following drawbacks and inconveniences at the time of measuring their refraction characteristics.
(1) In a lens after its periphery is ground and fitted in the lens frame, of the various marks described with reference to FIG. 1, all the marks, excluding the diamond mark 11, the mark of additional degree of the near distance 14, and the manufacturer's mark 17, are erased. As a result, when an attempt is made to later measure the refraction characteristics of the lens fitted in its frame, it is utterly impossible to discern from its appearance alone at which positions the far-distance or near-distance refraction characteristics should be measured. In consequence, it is necessary to find specified measuring positions while viewing through the lens meter on a trial-and-error basis.
(2) As a measure against the problem described in (1) above, lens manufacturers employ check cards. Specifically, a method is adopted whereby a card illustrating various marks shown in FIG. 1 is prepared, and, by matching the diamond mark on the check card with the diamond mark 11 left on the lens fitted in the frame, relevant marks are written on the mark with a marker pen or the like with reference to the mark indicating a far- or near-distance refraction characteristics measuring portion provided on the check card, and measurement is effected.
However, these check cards are not necessarily stored in every eyeglass store, and the work per se has been very complicated.
(3) Furthermore, in order to know the name of the manufacturer of the framed lens, the manufacturer's mark 17 may be checked. In addition, to position the lens with the check card, the diamond mark 11 may be checked, and if it is desirous to know the additional degree of the near distance alone, the indicator mark 14 may be searched on the lens, respectively. However, since these marks are very small and are indicated vaguely, the retrieval per se is extremely difficult in the present situation.